Heave stabilization of semi-submersible platforms

Abstract

Semi-submersible offshore drilling platforms are stabilized to reduce heave at wave periods which are shorter than those at which heave resonance occurs. Platform structures include supporting columns and pontoons. The pontoons extend outboard of the columns and have non-uniform cross sections. In addition, the pontoons may be shifted outwardly from the column center-lines. Reduced heave response to head-on, stern-on and beam waves is achieved.

I claim: 
     
       1. A semi-submersible floating structure comprising: deck means;   four columns attached to and extending below said deck means near the extremes thereof for submersion in water to provide buoyant supporting force for the structure, each of the columns having an effective center of gravity of displacement;   pontoon means including a first pontoon attached to a first pair of said columns near the lower ends thereof that are adapted to be submerged and a second pontoon attached to a second pair of columns near the lower ends thereof that are adapted to be submerged, said pontoon means having upper horizontal surface projections which are positioned to remain submerged and having portions which extend longitudinally from the associated column with greater cross-sectional area than the portions thereof between columns.   
     
     
       2. A semi-submersible floating structure as in claim 1 wherein said first and second pontoons are attached to the respective columns is laterally outwardly-shifted relationship to the centers thereof.